Alloy based on iron, containing nickel, chromium and aluminium, and process for obtaining same



Aprll 28, 1964 I. BEHAR 3,131,055

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, ANDPROCESS FOR OBTAINING SAME Filed March 6. 1961 4 Sheets-Sheet 1 FIg.'l

I. l I I I I I I I I I B 0 T 7 I -I "I I l I I I I I i II I I l [I l I:l

l I I II I I I I II I I 1 I AI I 1 I l I l I c c c c AAI April 28,1964 1. BEHAR 3,

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, ANDPROCESS FOR OBTAINING SAME Filed March 6, 1961 4 Sheets-Sheet 2 Fig.2

Apnl 28, 1964 l. BEHAR 3,

ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ALUMINIUM, ANDPROCESS FOR OBTAINING SAME Filed March 6, 1961 4 Sheets-Sheet 5 l. BEHAR3,131,055 ALLOY BASED ON IRON, CONTAINING NICKEL, CHROMIUM AND ANDPROCESS FOR OBTAINING SAME 4 Sheets-Sheet 4 L m 9 M 9 1 h com 2 1 M p AUnited States Patent M The present invention relates to alloys based oniron, nickel, chrome and aluminium and has for an object such an alloywhich is rich in iron. The invention also relates to a process forobtaining such an alloy.

Another object of the invention is the manufacture of an alloy which canbe worked when cold or hot and which, after the final hardeningtreatment, has constant physical characteristics from one casting toanother and, in particular, a particularly high elastic limit andbreaking strain.

In industrial practice hardening processes of different types are used.The hardening of steels which are rich in carbon is known; in this casethe austenite is decom posed in the course of cooling to give rise toharder structures (martensite, bainite). The hardness of the products ofdecomposition of the austenite in the course of cooling is higher whenthe carbon content is greater. This type of hardening, which allows highmechanical characteristics to be obtained, has, however, a certainnumber of drawbacks, such as the deformation resulting from hardeningfrom a high temperature, irregularity in its characteristics, therelative fragility of carbon-rich steels, or welding dhliculties, forexample.

The structural hardening of stable austenites is also known, thedevelopment of which is a result of research undertaken since 1928. Thistype of hardening has shown itself to be particularly flexible, it canbe carried out by tempering from a relatively low temperature and thisallows pronounced deformations and the risks of ordinary hardening to beavoided. However, the degree of hardness obtained by structuralhardening of the stable austenite is much smaller than that which can beobtained by the hardening of steels.

Certain authors have studied the structural hardening of stable ferriticalloys; in this case also the hardening is not very great and, apartfrom this, these alloys are relatively fragile.

The structural hardening of stable austenites results from the variationin the solubility of the hardening elements as a function of thetemperature, and I have determined that a change in the crystallographicstructure of the alloy permits structural hardening in general andstructural hardening by aluminium in particular to be raised.

Reference will now be made to the accompanying drawings showingrepresentative graphs, and in which:

FIGURE 1 shows curves representing the different solubilities ofaluminium under different conditions,

FIGURE 2 shows the variations in the Vickers H hardnesses for differentkinds of castings,

FIGURE 3 shows a graph of proportions of alloyed nickel and chromium,and

FIGURE 4 shows the variation of the mechanical characteristics of thealloy when hot as a function of the temperature.

In order to define the nature of the alloy, according to the invention,reference is now made to FIGURE 1 which shows diagrammatically thevariation in the solubility of aluminium in a stable austeniticstructure (curve E) and in an alloy having a gamma-alpha transformationon cooling (curve EDBA) at the temperature T If the stable austeniticalloy and the alloy having a gamma-alpha 3,131,055 Patented Apr. 28,1964 transformation upon cooling are both quenched from the temperatureT and then aged at the temperature T the hardening will be proportionalto the length C C in the case of austenite and C C' in the case of thealloy which has a gamma-alpha transformation. Referring now to FIGURE 2,the curves 1, 2, 3 and 4 represent the variations in the Vickers Hhardnesses determined at 2.5 kilograms, as a function of the temperingtemperature for 3 hours for three stable austenitic castings and for onecasting which has a gamma-alpha transformation upon cooling, thecompositions of which are given in the following table.

C Si Mn Ni C1 A1 The hardness of these alloys have been determined inthe quenched state, after an intense cold-working of 87.5% followed byageing carried out in the cold-worked state. It was observed that thestructural hardening of the casting 4 upon ageing at 450 C. to 500 C. isparticularly great. The alloy according to the invention is thereforeone which has a gammaalpha transformation upon cooling in such a waythat the hardening element is dissolved at a high temperature in thegamma state and then precipitated in the alpha phase after thegammaalpha transformation.

According to the invention the proportions by weight in the alloy ofnickel and chromium correspond to the area defined by the outline ABCDEshown in FIGURE 3 of the accompanying drawing and the alloy which isfree from 6 ferrite has, moreover, a proportion by weight of aluminiumranging from 1% to 6% and a content of carbon at the most equal to0.15%, this alloy being subjected on the one hand to an annealingtreatment followed by quenching which transforms the greatest part ofaustenite into an a structure of martensitic appearance and leavesalmost all the aluminium in super-saturation and on the other hand to ahardening tempering precipitating a compound which is rich in aluminiuminto the alloy.

The thermal treatment conditions leaving the aluminium insuper-saturation or, in a general way, leading to a softened state asagainst a hardened state will hereinafter he referred to assuper-hardening.

The alloy transformed by the super-hardening can afterwards be hardenedby tempering at a sufficiently low temperature which does not involveoxidation or deformation.

An essential characteristic of the alloy according to the invention isthat it does not contain free 6 ferrite and that by reason of therelation of the proportions between the alpha-producing elements and thegamma-producing elements, this ferrite cannot appear in the course ofthermal treatments. In these conditions the property of aluminium tofavour the transformation of the austenite into on ferrite ofmartensitic appearance and the property of alu minium of being veryslightly soluble in its a phase, whilst it is very soluble in its 7phase, can be fully used.

In the course of heating at high temperature the alloy is austenitic andit is thus possible to cause a considerable proportion of aluminium topass into solid solution. As this autenite is not stable it istransformed into or ferrite of martensitic appearance in the course ofcooling.

The dissolving or softening treatment is effected at a temperatureextending from 750 to 1250 C. The duration of maintenance is variableaccording to the temperature and the size .of the pieces to be treated.It amounts to a few minutes in the case of transitory treatments at atemperature of 1150 C. for example and can extend to 20 hours when thetreatment is at a relatively low temperature. This softening can becarried out in several stages: It can for example be convenient to carryout a standardized treatment at a high temperature and then a softeningtreatment at a lower temperature. Usually a treatment of 30 minutes at800 C. is sufficient.

The cooling which follows the dissolving can take place, as desired, inair, oil or water.

The hardening tempering can he carried out at a temperature ranging from200 C. to 700 C. according to the desired characteristics and accordingto the treatment to which the alloy has been subjected before tempering(super-hardening or super-hardening followed by cold working). Theduration of the tempering can be contained between a few minutes and 24hours. The hardening can also be carried out by repeated ageingtreatments, each one of these treatments being followed by cooling atthe ambient temperature or at a lower temperature.

During this tempering the aluminium originally in super-saturation inthe a ferrite of martensitic appearance formed in the course of cooling,is precipitated in the martensite in the form of an aluminium-richcompound producting a high degree of structural hardening.

The hatched area defined by the outline ABCDE in FIGURE 3 determines theproportions of nickel and chromium which can be used according to theinvention, the representative point of the alloy being found ofnecessity inside this outline. In the drawing, the abscissa indicatesthe chromium constituent and the ordinate indicates the nickelconstituent.

The alloy according to the invention contains moreover 1% to 6% ofaluminium which can be partially replaced by titanium and/ ormolybdenum, the aluminium content not going below 0.5%. The addition oftitanium and/ or molybdenum can have as its efiect the modification ofthe composition, the crystallographic structure or the kinetic ofprecipitation of the aluminium-rich compound.

The alloy can also contain up to 2% silicon and up to 4% manganese.

The carbon content must be limited at the most to 0.155%.

Bearing in mind the influence of the different elements of the alloy onthe formation of 6 ferrite, it is also necessary that the sum of thepercentages by Weight Cr+1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi12 C0.2 Mn shouldat the most be equal to 10.

Furthermore the alloy can contain different additions in smallquantities which however do not modify its nature so much regarding thebalance of the alpha- 'producing and gamma-producing elements asregarding the mechanism of the structural hardening of the alloy byprecipitation of an aluminium-rich compound and/or a titanium-richcompound and/or a molybdenum-rich compound in the course of hardeningtempering.

The alloys according to the invention have a greater amenability tohardening and a greater elastic limit, due to the precipitation in theon phase of an aluminium-rich and/ or titanium-rich and/ ormolybdenum-rich constituent, than the alloys which are free from theseelements such as alloys with 18% of chromium and 8% nickel.

Certain known alloys assaying about 17% of chromium and 7% of nickelcontain a relatively small proportion of aluminium. These alloys havefree 6 ferrite. The alloys according to the invention, by reason of thebalance of the chromium and nickel elements defined by the hatched areaABCDE, do not contain 6 ferrite. They therefore have a higher aluminiumcontent which can be contained between 1% and 6% and is generally higherthan 2.5%, when the highest mechanical characteristics are to beobtained.

The absence of ferrite is extremely important in this type of alloy.

The austenite of the alloy which is very unstable is decomposed in the aphase of martensitic appearance in the course of cooling as far as theambient temperature which follows the treatment at high temperature: theresidual austenite, if there is any, can be transformed by refrigerationat a lower temperature or by cold working. The hardening of the alloybeing connected with the variation of solubility of the aluminium in the'y and a phase and the composition of the alloy being such that free 6ferrite does not appear, all the matrix can participate in thestructural hardening due to the precipitation of an aluminium-richcompound in super-saturation in the o: ferrite. The existence of acertain quantity of free 6 ferrite can therefore in certain known alloysreduce the possibility of structural hardening.

As aluminium, apart from its action on the amenability to hardening,favours the formation of 6 ferrite, it is essential that the chromiumand nickel contents be contained within the hatched area ABCDE and thatthe sum of the percentages by weight Cr+ 1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi12C-0.2 Mn be at the most equal to 10 in order to avoid the risks offormation of 6 ferrite. In the case of known alloys, assaying about 17%of chromium, 7% of nickel and 1.2% of aluminium, small variations in thecomposition of the castings can bring about considerable variations inthe free 8 ferrite content, and, as a result, a divergence in themechanical characteristics from one casting to another.

Among the other advantages of the claimed alloy, in connection with theabsence of 6 ferrite, may be mentioned the following:

The super-hardening temperature can be raised in order to dissolve agreater aluminium content without the risk of producing a highproportion of free ferrite.

The absence of the risk of formation of 6 ferrite allows thesuper-hardening temperature to he fixed Without any high degree ofprecision. In the alloys containing a certain proportion of 6 ferrite onthe other hand a variation in the softening temperature would bringabout an important variation in the free 8 ferrite content and, as aresult, variations in the mechanical properties of the alloy.

The absence of the formation of 6 ferrite in the alloy according to theinvention allows the drawbacks related to the dendritic segregation tobe reduced. In the alloys containing 6 ferrite these drawbacks areparticularly accentuated when the ingots are of large dimensions, thecentre of the ingots being then very rich in ferrite.

The alloy having at high temperatures, an austenitic structure free of 6ferrite, the transformations of the alloy when hot (forging and rolling)do not present any difficulty.

To sum up, an absence of free 5 ferrite in the alloy according to theinvention allows structural hardening of the alloy to be raised, allowsa good reproducibility of the mechanical characteristics from onecasting to another to be obtained, allows the drawbacks connected withthe dendritic segregation to be reduced, and allows the transformationof an alloy in the hot state to be facilitated.

The alpha phase obtained after super-hardening is plastic enough for thealloy to be easily moulded, cold rolled, drawn or swaged as the results,recorded in the following examples, show.

The invention will now be described with reference to castings ofdifferent compositions and with different degrees of treatment. Thecastings are preferably melted in vacuum, but it is also possible tomelt them in air.

In the examples which follow, indications will be given concerning thepossibility of replacing certain elements of the alloy by equivalentelements.

Example 1 A casting was made having, in percentage by weight, thefollowing composition:

C 0.025 Cr 7.01 Si 0.14 Al 3.69 Mn 0.59 Fe to The treatments andcharacteristics of the alloy are summarized in the following table:

The super-hardening treatments were carried out on pieces of 7millimetres in diameter and 65 millimetres long and the traction testpieces were taken from among forged plates.

E is the 0.2% yield strength.

R is the ultimate tensile strength.

A is the elongation.

and 2 is the contraction of cross-section.

Example 2 A casting was carried out having the following composition:

C 0.03 Cr 10.91

Si 0.14 Al 2.03 Mn 0.53 Fe to 100.

The treatments and characteristics of the alloy are summarized in thefollowing table:

Treatments E 0.2, R, 2,

kg/n'nn. kgJmm. percent percent 30 111111., 800 Oil-cooled 65.1 103. 511.5 74.1 30 min., 800+480, 1 hour air-cooled 134. 1 149. 5 12. 9 47. 130 min., 800+500, 1 hour air-cooled 146. 2 160. 1 10. 4 51.9 30 min,800+550, 1 hour air-cooled 130. 7 147.0 11.0 45. 2

E, R, A and 2 being as before. It is noted that the replacement of aportion of the aluminium weight for weight by titanium and/ ormolybdenum gives alloys having important properties, the aluminiumcontent being at least equal to 0.5%.

Example 3 By way of example there are hereafter indicated the mechanicalcharacteristics obtained with a casting E, R, A and 2 being as before.

Example 4 It has been determined that the replacement of a portion ofthe chromium weight for weight by molybdenum, always with a maximummolybdenum content in the alloy .of 4.5%, allows the mechanicalcharacteristics when cold to be increased. Apart from this themolybdenum allows the benefit of hardening at a moderately hightemperature to be retained. A casting was carried out having thefollowing composition:

C 0.01 Cr n 4.36 Si 0.14 Al 3.51

Mn 0.57 Mo 2.40 Ni 15.10 Fe to 100.

This casting is derived from that of Example 1 by replacing about 2.5%of the chromium by 2.5 of

molybdenum.

Treatments E 0.2, R, A, 2,

kg./mm. kg./mm. percent percent 30 min, 800 oil cooled 108. 0 126. 8 9.071. 9 30 111111., 800+3 hr., 500 aircooled 182. 9 194. 7 8. 1 26. 7 30min., 800+3 hr., 520 aircooled 186. 1 194. 8 9. 2 42. 1 30 min., 800+3hr., 550 aircooled 177. 6 184. 2 7. 9 51.0 30 min.,'800+3 hr., 58Laircool (1 170.8 175.4 7.2 52.8

E, R, A and 2 being as before.

The characteristics obtained after a super-hardening followed bytempering are very clearly superior to those obtained on alloys hardenedby aluminium, assaying about 15% of chromium, 7% of nickel and 2% ofmolybdenum.

It should be noted that after quenching from 800 C. and tempering for 3hours at 550, it is possible to obtain an elongation of 9% and areduction of area of 40% whilst the ultimate tensile strength is 195kilograms per square millimetre and the 0.2% yield strength is 185kilograms per square millimetre. The corresponding resilience is K =2.2kilograms per square centimetre whilst the resilience of the alloysassaying about 15% of chromium, 7% of nickel, 2% of molybdenum and 1.2%of aluminium is lower than 1 and generally of the order of 0.5 kilogramper square centimetre, for a tensile strength of the order of kilograms,per square millimetre.

The technical superiority of the alloy of the present invention over theknown alloys derived from alloys with 18% of chromium and 8% of nickeland hardened by aluminium, clearly appears in the curves in FIGURE 4which represent the variation of the mechanical char acteristics whenhot as a function of the temperature at which the rapid traction testwas carried out.

In this figure, the characteristics of the reference alloy are indicatedin unbroken lines While the characteristics of the alloy according tothe invention are indicated in broken lines.

The reference alloy had the following composition:

C 0. 08 Cr 15.10 Si 0.47 Al 1.30 'Mn 0.76 Mo 2.60 Ni 7.18 Fe to 100.

The test pieces were subjected to the following treatment beforetraction tests:

1 hour at 1050 C. oil-cooled+1 /z hours at 760 C., Water-cooled at 0C.+maintained for /2 hour at 0 C.+1 hour at 570 C., air-cooled.

The alloy according ,to the invention. had the following composition:

C 0.025 Cr 4.17 Si 0.01 A1 3.28 Mn 0.57 Mo 2.20 Ni 16.2 Feto 100.

Before the hot traction tests, the test pieces were treated for 3 hoursat 800 C., oil-cooled+3hours at 550 C.

air-cooled.

It is pointed out that at every temperature the 0.2% yield strength andthe tensile strength of the alloy according to the invention are clearlyhigher and that the percent elongation and the percent reduction of areaare equal to or higher than those of the reference alloy.

Creep tests confirm the technical superiority of the alloy according tothe invention.

The creep tests were carried out at 400 C. using the same castings andthe same thermal treatments as those used for the rapid traction testsabove room temperature.

Reference alloy Alloy oi the invention Longitu- Load, kg./ruu1. dinalLongitu- Liie in expansion Life in dinal hours up to hours expansionbreaking, to breaking,

percent percent It is also possible to replace a portion of the chromiumweight for weight by molybdenum. In this case abscissa of FIGURE 3corresponds to the sum Cr-l-Mo, molybdenum being added in amounts equalto or less than 4.5%.

Similarly it is possible to replace a portion of the nickel by manganeseand by carbon in quantities so that the sum Ni% +30 (3% +0.5 Mn% isequal to the nickel content corresponding to the extent defined by theoutline ABCDE in FIGURE 3. In any case the manganese content must notexceed 4% and the carbon content must not exceed 0.15%.

In the case of the substitution of the chromium by molybdenum and/ oraluminium and of the nickel by carbon and/or manganese, it is necessarythat the sum Cr+1.5 Si+2.5 Al+2.5 Ti+0.8 MoNi-12 C-0.2 Mn remains lowerthan 10.

The alloy according to the invention can be plastical- 1y deformed whencold without difficulty before final hardening. This property can bemade use of in particular for obtaining metal foil and wire.

It is possible to obtain a reduction in thickness of more than 90% byrolling when cold and of more than 75% by drawing without anyintermediary softening.

It is also possible to obtain deep swaging on the superhardened metal.

For example on a blank of 0.24 millimetre thickness transientlysuper-hardened at 1150 C. and having the following composition:

C 0.04 Cr 7.03 Si 0.01 Al 3.54 Mn 0.47 Fe to 100.

the swaging depth carried out in the course of an Erichsen test was 9millimetres, the diameter of the ball being 20 millimetres and thediameter of the jaws 27 millimetres.

It was also determined that if an intense cold-working was carried outafter super-hardening, a tempering treatment afterwards carried out at amean temperature which is often of the order of 400500 C. considerablyraised the hardening of the alloy.

By way of example a cold-working of 87.5% by rolling when cold and atempering of 3 hours at 450 C. allows there to be obtained, on a foil of0.10 millimetre having the same composition as that of the plate of 0.24millimetre which has just been mentioned, an elastic limit at 0.2% of240 kilograms per square millimetre and a breaking strain of 250kilograms per square millimetre.

On a wire of the same casting, cold worked by drawing to 73%, thecharacteristics obtained after ageing for 3 hours at 450 C. are 230kilograms per square millimetre for the conventional elastic limit at0.2% and of 238 kilograms per square millimetre for the breaking strain.

The particularly high mechanical characteristics, obtained aftersuper-hardening and tempering or after superhardening, cold working andtempering of an alloy according to the invention, allow it to be used inthe field of the manufacture of springs and in a general way in thefield of aeronautics and ballistic machines. With regard to this it isinteresting to note that the density of the alloy is relatively small,of the order of 7.5 and that the Elastic limit Density relation istherefore particularly high.

The alloy according to the invention has moreover a good resistance tocorrosion. Taking into account the nickel, chromium and aluminiumcontents, the alloy does not rust in water and is not attacked byimmersion in a 3% solution of sodium chloride or in solutions, atvariable concentrations, of acetic acid.

Apart from this the chromium-rich alloys corresponding to the areadefined by the outline ABCDE have a good resistance to solutions ofnitric acid.

Five consecutive tests each of 48 hours were carried out at the ambienttemperature on an alloy having the composition:

C 0.04 Cr 11.87 Si Traces Al 3.59 Mn 0.31 Fe to 100.

The breaking strain after cold working and hardening tempering at 500 C.were 241 kilograms per square millimetre.

The resistance to corrosion is important 'for many applicat-ions and inparticular the alloy according to the invention is very suitable for themanufacture of motor springs intended for watch making, where corrosionis the forerunner of breakage.

Because of the high content of nickel, chromium and aluminium, an alloyaccording to the invention does not rust if it is exposed to theatmosphere.

A high quality motor spring must also develop a high torque and thisimplies good mechanical characteristics. These high mechanicalcharacteristics are obtained by intense cold working followed bytempering at a relatively low temperature, generally between 400 C. and550 C.

Bearing in mind the hardening process of the alloy, the manufacture ofmotor springs for watchmaking is more simple than that of carbon-steelsprings. Apart from this, springs manufactured from this alloy havecertain advantages over springs manufactured of carbonsteel:

(1) It is not necessary to take particular precautions to avoidcorrosion either for the manufacture or for the stocking or despatchingof the springs, the alloy being substantially un-tarnishable.

(2) The manufacturing process requires only a tempering on a previouslycold worked material, the treatment furnaces are simple and thecharacteristics obtained with the springs can be very regular. In thecase of carbon-steels, on the other hand, hardening installations arenecessary and the eiiiciency of the hardening is never very regular.

(3) Since the cold worked alloy retains a good malleability thefashioning of the springs is easy and does not involve an abnormal Wearin tools.

It is also unnecessary to soften one of the ends of the spring in orderto form the retaining eye. After the hardening treatment this eyethere-fore remains rigid and does not deform in service in the watch.

(4) Since the carbon content of the alloy remains small the shackle canbe spot welded without difliculty.

(5) Springs manufactured from the alloy also have the followingadvantages:

(a) They are unbreakable in the watchm-aking sense.

(b) They develop high torques.

(0) They are stable. The torques of the springs do not decrease due tofatigue nor upon prolonged maintenance at the ambient temperaturecontrarily to that which is observed in the case of carbon-steelsprings.

The cold working necessary to develop the hardening of the alloy can becarried out in different ways according to the transformation processused. A. The alloy is transformed into bands by cold rolling.- The coldrolled bands are strongly cold worked by cold rolling: according to thethickness of the original hot rolled bands and the thickness of thefinal bands one or more intermediary softening treatments may benecessary in order to carry out a determined cold-working rate which ismost often higher than 80%.

B. The alloy is hot rolled in the machine wire state. After calibratingthe machine wire by drawing the latter is rolled cold in order to obtainsmall hands, the width of which, which depends upon the diameter of theoriginal machine wire, is often greater than millimetres. One or moreintermediary softenings may be necessary in order to achieve an adequatecold working rate which is often higher than 80%.

In these two transformation processes the eyes necessary for themanufacture of springs are clipped into the band or the litle band andthe edges are mechanically polished.

C. The alloy is hot rolled in the machine wire state. Instead ofattempting to calibrate the wire it is possible to carry out the drawingprocess until a wire is obtained the diameter of which is such that byfinal rolling the necessary strip is obtained for the manufacture of thespring. Since the wire having small diameter and the cold working byrolling are then insufiicient it is necessary in order to obtain goodcharacteristics in the springs to combine the cold working by drawingand cold working by rolling.

Whatever the transformation technique used, the alloy according to theinvention allows good motor springs to be obtained and retains theproperties mentioned above.

I claim:

1. An alloy consisting essentially of iron, nickel, chromium andaluminium, the proportion by weight in the alloy of nickel and chromiumcorresponding to the area ABCDE in the FIG. 3 of the accompanyingdrawings, the aluminium content by weight being of 1 to 6% and themaximum carbon content being 0.15% including as possible components Si,Ti, Mo and Mn in which the sum Cr+1.5 Si+2.5 Al-|-2.5 Ti+0.8 Mo-Ni12C0.2 Mn is at the most equal to 10, the balance being iron, said alloybeing free of 6 ferrite and susceptible to annealing at 750 C. to 1250C. and quenching through phase transformation to a substantiallyaluminium-soluble martensite-like structure and thereafter toprecipitation hardening at from 200 C. to 700 C. by an aluminium richcompound, the component Ni+30 C+0.5 Mn being sub stitutable for Ni andthe component Cr-l-Mo being substitutable for Cr, Mn then having amaximum value of 4% and Mo having a maximum value of 4.5%

2. An alloy as described in claim 1, in which the aluminium is partiallyreplaced by a metal selected from the group consisting of titanium,molybdenum and mixtures thereof, the aluminium content remaining atleast equal to 0.5%.

3. An alloy as described in claim 1, including a maximum of 2% siliconby weight.

4. An alloy as described in claim 1, including a maximum of 4% manganeseby Weight.

5. A process for hardening an alloy based on iron, including the stepsof forming an alloy free of 5 ferrite as described in claim 1, thenannealing the alloy at about 750 C. to 1250 C., then quenching the samethus achieving a soft substantially aluminium-soluble martensite-likestructure and then ageing the alloy at least once at a temperatureranging from 200 to 700 C. to precipitate therein an aluminium richcompound.

6. A process as described in claim 5, in which the quenching is carriedout in a medium selected from the group consisting of air, oil andwater.

7. A process as described in claim 5, in which the tempering is precededby cold working.

References Cited in the file of this patent UNITED STATES PATENTS1,538,360 Smith May 19, 1925 1,941,648 Armstrong Jan. 2, 1934 2,048,164Filling et a1. July 21, 1936 2,505,762 Goller May 2, 1950 FOREIGNPATENTS 378,478 Great Britain Aug. 15, 1932 404,876 Great Britain J an.25, 1934

1. AN ALLOY CONSISTING ESSENTIALLY OF IRON, NICKEL, CHROLMIUM ANDALUMINIUM, THE PROPORTION BY WEIGHTIN THE ALLOY OF NICKEL AND CHROMIUMCORRESPONDING TO THE AREA ABCDE IN THE FIG. 3 OF THE ACCOMPANYINGDRAWINGS, THE ALUMINIUM CONTENT BY WEIGHT BEING OF 1 TO 6% AND THEMAXIMUM CARBON CONTENT BEING 0.15% INCLUDING AS POSSIBLE COMPONENTS SI,TI, MO AND MN IN WHICH THE SUM CR+1.5 SI+2.5 AL+2.5 TI+ 098 MO-NI-12C-0.2 MN IS AT THE MOST EQUAL TO 10, THE BALANCE BEING IRON, SAID ALLOYBEING FREE OF $ FERRITE AND SUSCEPTIBLE TO ANNEALING AT 750*C. TO1250*C. AND QUENCHING THROUGH PHASE TRANSFORMATION TO A SUBSTANTIALLYALUMINUM-SOLUBEL MARTENSITE-LIKE STRCTURE AND THEREAFTER TOPRECIPITATION HARDENING AT FROM 200*C. TO 700*C. BY AN ALUMINIUM RICHCOMPOUND, THE COMPONENT NI+30 C+0.5 MN BEING SUBSTITUTABLE FOR NI ANDTHE COMPONENT CR+MO BEING SUBSTITUTABLE FOR CR, MN THEN HAVING A MAXIMUMVALUE OF 4% AND MO HAVING A MAXIMUM VALUE OF 4.5%.